1. Field of the Invention
This invention relates to siloxane elastomers containing sulfur and methods of their preparation.
2. Description of the Prior Art
Compositions containing mercaptoorgano functional siloxanes and aliphatically unsaturated organosiloxanes have been described in the prior art. For example, Viventi in U.S. Pat. No. 3,816,282 teaches silicone rubber compositions which cure at room temperature in the presence of electromagnetic and particulate radiation. The compositions exposed to the radiation for curing comprise an organopolysiloxane having a viscosity of 100 to 100,000 centipoise at 25.degree. C., 0.1 to 2.0 mole percent of the organic groups as silicon-bonded vinyl radicals and from 1.98 to 2.05 organic groups per silicon atom, an organopolysiloxane fluid having the formula ##EQU1## WHERE R.sup.3 is alkyl or aryl of up to 18 carbon atoms, c is 1 to 25, a is from 0.088 to 2.08, b is from 0.009 to 0.97 and a + b is from 2.019 to 2.16 and the number of mercapto siloxane units equal or exceed the number of non mercapto siloxane units, and from 0.025% to 1% by weight of the composition of a free radical source which includes certain organic peroxides.
Bazant et al. in German Patent Publication (OLS) No. 2,008,426 discloses five different possibilities to make three dimensionally crosslinked silicone polymers. These reactions were found to proceed in the presence of radical reaction initiators at a temperature of 30.degree.-110.degree. C. or by UV light initiation. The five possibilities are defined as follows: One possibility is a reaction between alkenylsiloxanes of the general formula ##STR1## where m + l is greater than 30, l:m is from 1:10 to 1:60, x is 0 to 4 and R.sup.4 is alkyl groups of 1 to 6 carbon atoms, cycloalkyl groups of 5 to 8 carbon atoms, aryl groups or siloxy groups, and dithiols of the general formula ##STR2## where p is 0 to 30, x is 1 to 4 and R.sup.5 is alkyl groups of 1 to 6 carbon atoms, cycloalkyl groups of 5 to 8 carbon atoms or aryl groups.
The second possibility is reactions between alkenylsiloxanes of the general formula I and high molecular thioalkylpolysiloxanes of the general formula ##STR3## where p + q is greater than 30, q:p is 1:10 to 1:60, R.sup.5 is defined above and R.sup.6 is an alkylene group of 1 to 6 carbon atoms, a cycloalkylene group of 5 to 8 carbon atoms or an arylene group.
The third possibility is reactions between the thioalkylpolysiloxanes of the general formula III and low molecular alkenylsiloxanes of the general formula ##STR4## where r is 0 to 30, x is 0 to 4 and R.sup.5 is defined above.
The fourth possibility is reactions of high molecular thioalkyl- and alkenyl substituted siloxanes of the general formula ##STR5## R.sup.5 and R.sup.6 are defined above.
The fifth possibility is reactions between thioalkylpolysiloxanes and organic diisocyanates.
George A. Gant in U.S. patent application Ser. No. 401,791, filed Sept. 28, 1973, entitled "UV Curable Compositions" and assigned to the same assignee as the present application, discloses a composition which is curable with ultraviolet light consisting essentially of a siloxane having 0.1 to 100 mole percent mercapto functional siloxane units of the formula ##EQU2## and any other siloxane unit being of the formula ##EQU3## where x is 0 to 2, y is 1 to 2, x + y is 1 to 3, z is 0 to 3, R.sup.7 is an alkylene radical and R.sup.8 is a monovalent hydrocarbon radical or a monovalent halogenated hydrocarbon radical, a siloxane having 0.1 to 100 mole percent of vinyl siloxane units of the formula ##EQU4## where l is 0 to 2 and R.sup.8 is defined above and any other non-vinyl containing siloxane units are defined by formula V, and a photosensitizing amount of a photosensitizer, where the ratio of vinyl to mercapto is from 1:100 to 100:1.
Although the prior art describes that compositions containing mercapto functional siloxanes and vinyl containing siloxanes can be cured and that some compositions cure to elastomeric products, it is not obvious that certain compositions can be cured at room temperature as well as with heat with organic peroxide alone. For example, Viventi requires specific mercapto containing siloxanes and radiation for curing his composition, Bazant et al. place strict limitations on the type of alkenyl siloxanes and mercapto siloxanes which can be combined to provide a three dimensional crosslinked siloxane and finally Gant requires the presence of a photosensitizer and ultraviolet light to cure his compositions. Thus, it was unexpected that low molecular weight alkenyl containing polysiloxanes and a class of mercaptoorganopolysiloxanes could be cured with organic peroxide at both room temperature and by heating.